The present invention relates generally to overvoltage protectors and more particularly to a device using a varistor initiated arc as an overvoltage protector.
Electronic circuits under field environments are often subjected to undesired transient high voltage signals known as overvoltages. For example, equipment which is either subject to lightning strikes or is connected to antennas subject to lightning strikes must be protected against overvoltages that can cause component burnout, malfunction or premature activation. To achieve this protection, many devices using a varistor material to discharge the overvoltage have been developed.
U.S. Pat. No. 2,072,850 of G. E. Andre discloses a device to protect radio antennas from lightning using at least two conductors having a gap therebetween filled with a compacted, granular material such as silicon carbide. According to the disclosure, the spacing of the conductors and the compression of the material are the factors which control the magnitude of the overvoltage that will be conducted.
Other devices have been constructed in the form of electrical connectors which permit electrical signals to be communicated along cables in a conventional manner, but which discharge overvoltages to ground.
U.S. Pat. No. 3,702,420 of J. A. Cooper discloses an electrical connector having a plurality of conducting pins insulated from ground by a thin sleeve of high dielectric constant material that terminates at an air chamber within the connector. An overvoltage on the pin is stimulated by the dielectric material to arc to ground in the air chamber at the end of the sleeve.
U.S. Pat. No. 3,711,794 to D. Tasca et al. discloses a coaxial connector which utilized a toroidal shaped member of varistor material comprising fine particles of zinc oxide and additives that have been pressed and sintered at high temperatures to provide a composite body of metallized material. As the voltage across the material increases, its impedance decreases, effectively limiting the overvoltage on the center conductor of the connector.
U.S. Pat. No. 3,725,745 of W. J. Zisa discloses an overvoltage protector for an electrical watt-meter including a compacted mass of granulated silicon carbide in series with an air spark gap. This invention uses the silicon carbide to limit the current which flows when an overvoltage causes a spark to jump the gap, thus minimizing electrode splattering caused by high current sparks.
These and other similar previous devices all have certain disadvantages which minimize their effectiveness when protecting certain electronic equipments from overvoltages caused by lightning strikes. Many spark gap devices do not break down at a low enough voltage to protect sensitive semiconductor components. Those designed to break down at lower voltages, such as are shown in the Cooper patent, are not predictable enough in their performance; i.e., variations in breakdown voltage caused by manufacturing tolerances or changes in humidity keep them from being as reliable as desired for many applications. Similarly, devices which depend on conduction of a varistor material, such as shown in the other patents noted above, may work satisfactorily at low voltages but be destroyed by the high current of a lightning strike.